This invention relates to a scissor lift mechanism having at least two scissor elements connected in pairs by a swivel axis, so that the swivel axis is arranged between the respective end sections of the scissor elements; and having a drive for raising and lowering the scissor elements by a traction member.
A scissor lift mechanism of this type is described, for example, in published German Patent application no. DE 100 24 075 A1. Through the traction member, the distance between the upper and lower ends of the scissor elements is varied. The traction member is therefore connected to one end of one scissor element and to the drive via a pulley on an upper end of the other scissor element and therefore it can be drawn in. In this case, the traction member acts on an expansion body which in the lowered resting position of the scissor lift mechanism is situated close to the lower end of a scissor element between it and the upper end of the other scissor element resting thereon. By applying the required tractive force, the expansion body is moved in the direction of the swivel axis. The angle formed by the scissor elements thereby becomes progressively larger so that the height of lift is varied accordingly. This traction member is not reeled up by this drive but instead it has an end section that is closed to form a loop to thereby prevent the traction member from slipping off.
It has proven to be a disadvantage with such scissor lift mechanisms that their height of lift is varied by an expansion body; that the lever ratios in the lowered resting position vary greatly from those in the raised working position. The forces required to change the height of lift therefore differ many-fold in the two extreme positions. In practice, this results in wear phenomena. In addition, a high driving power is therefore necessary. In addition, the movement of the expansion body must be adjusted using a complex control system, so the rate of lifting between the extreme positions is approximately constant but at least is not subject to any great fluctuations.
Another embodiment of a scissor lift mechanism is known from U.S. Pat. No. 4,534,544 (=DE 33 31 872 A1) in which the two end sections of the scissor elements are joined by a drive constructed as a hydraulic cylinder. However, this means a large opening path of the hydraulic cylinder, so that here again, the forces required to be applied may vary greatly. Furthermore, hydraulic cylinders have only a limited suitability in many practical applications because of the oil they release.
German Utility Model DE 298 03 330 U discloses a scissor lift mechanism which ensures that relatively high lifting forces can be generated with comparatively low driving forces even in the initial phase of the lifting operation due to favorable lever ratios. However, it is a disadvantage here that this results in two speed levels in the lifting and lowering operations and that a jerky change in speed occurs when the expansion body is gripped by a supporting swivel arm. In this case, the expansion body acts only according to the wedge principle on the upper and lower scissor-type elements before the intervention of the swivel arm, so that the upward movement proceeds very slowly. As soon as the expansion body has been raised by the supporting swivel arm, the result is an operative connection to only the upper scissor halves where the transport movement is accelerated suddenly. Precise positioning is therefore no longer possible in the transition area.
U.S. Pat. No. 4,858,888 also discloses a scissor lift mechanism in which the free end of the supporting arm is guided by a guide arm that is bent at an angle and is also attached to the first scissor element via an articulated connection.
Additional scissor lift mechanisms are known from WO 99/62813, DE 91 08 825 U, DE 35 02 641 A and U.S. Pat. No. 3,785,462.